1. Field of the Invention
This invention relates to a device for collecting a toner carrier in an image developing apparatus after an electrostatic latent image formed on a photosensitive material by, for example, electrophotography has been developed with a two-component magnetic brush.
2. Description of the Prior Art
A known device for collecting a toner carrier in an image developing apparatus is shown as a sectional view in FIG. 1. The developing apparatus has a main body 1 to which a developing bias is applied, and which contains a two-component developer not shown. The developer is composed of particles of a toner and particles of a magnetic carrier. A pair of stirring shafts 2 are provided in the main body 1 of the developing apparatus and are rotatable for stirring and mixing the two-component developer sufficiently and transporting it in two opposite directions which are perpendicular to the sheet of the drawing. The main body 1 has an opening in which the development of an image is carried out. A developing magnet roll 3 is rotatably disposed in the main body 1 adjacent to its opening and comprises a rotatable sleeve 3b which contains a plurality of stationary magnetic poles 3a. A doctor blade 4 is provided in the main body 1 near the developing magnet roll 3 for maintaining at a fixed level the quantity of the developer which is adsorbed by the surface of the roll 3 to form a magnetic brush, and thereby the length of the filaments of which the brush is composed. A photosensitive drum 5 is disposed in the opening of the main body 1 and has a surface covered with a layer of a photoconductive material which forms an electrostatic latent image when electrically charged and exposed to light by the electrophotographic (or Carlson) process. A carrier collecting magnet roll 6 is disposed in close proximity to the drum 5 and comprises an outer sleeve 6a and an inner magnet portion 6b which is rotatable. A scraper 7 is provided for contacting the sleeve 6a and scraping the carrier off its surface.
In operation, the two-component developer in the main body 1 of the developing apparatus is sufficiently stirred and mixed by the stirring shafts 2 and after it has been electrically charged, it is adsorbed to the surface of the sleeve 3b of the developing magnet roll 3 by the magnetic force of its stationary magnetic poles 3a and is conveyed upwardly with the rotation of the sleeve 3b in the direction of an arrow A in FIG. 1. The quantity of the developer adhering to the surface of the sleeve 3b is maintained at a uniform level by the doctor blade 4, so that the filaments of a magnetic brush formed by the developer on the surface of the sleeve 3b may keep a uniform length. As the roll 3 is rotated, the magnetic brush is rubbed against the surface of the photosensitive drum 5 and the toner adheres electrostatically to an electrostatic latent image which has been formed on the surface of the drum 5, whereby the image is developed. While the toner in the developer is transferred from the roll 3 to the surface of the drum 5, the carrier remains adherent to the roll 3 and is conveyed toward the stirring shafts 2. A part of the carrier, however, leaves the roll 3 by adhering to the drum 5, or scattering. The carrier adhering to the drum 5 becomes an obstacle to the subsequent process in which the toner image is transferred to recording paper, and causes a printing defect known as incomplete transfer. Therefore, the carrier adhering to the drum 5 is removed by the carrier collecting magnet roll 6 and the scraper 7 and is collected in the main body 1. The carrier on the photosensitive material is adsorbed to the surface of the sleeve 6a of the roll 6 by the magnetic force of its magnet portion 6b. When a cycle of developing operation is terminated, the magnet portion 6b of the roll 6 is rotated in the direction of an arrow B in FIG. 1 and the carrier is scraped off the sleeve 6a by the scraper 7, so that it may return into the main body 1.
A larger amount of carrier is, however, likely to scatter from the opposite ends of the magnetic brush on the developing magnet roll 3 beyond which no magnetic field exists, than from its middle portion. Accordingly, the carrier is more likely to gather at the opposite ends of the carrier collecting magnet roll 6 than in its middle portion. As the magnet portion 6b of the roll 6 does not extend to either end thereof, the carrier is less easy to convey about the opposite ends of the sleeve 6a than about its middle portion and can, therefore, be scraped off the sleeve 6a less effectively at its ends than in its middle portion.
A large amount of carrier is, therefore, likely to gather at the opposite ends of the carrier collecting magnet roll 6 and contact the conductor portion of the photosensitive drum 5, i.e. its portion not covered with the photoconductive material. The occurrence of any such contact results in the grounding of the developing bias which gives rise to a serious printing defect.